|  | 2.1 | Various LAN Communication Problems
| 2.1.5 | Network latency
Latency, sometimes called delay, is the time a frame (or a packet) takes to travel from the source station (node) to its final destination on the network. It is important to quantify the total latency of the path between source and destination for LANs and WANs. In the specific case of an Ethernet LAN, understanding latency and its effect on network timing is crucial to determining whether CSMA/CD for detecting collisions and negotiating transmissions will work properly. Latency has at least three sources. First, there is the time it takes the source NIC to place voltage pulses on the wire and the time it takes the receiving NIC to interpret these pulses. This is sometimes called NIC delay (it is typically around 1 microsecond for 10BASE-T NICs). Second, there is the actual propagation delay as the signal takes time -- albeit a very short time -- to actually travel down the cable (it is typically about .556 microseconds per 100 m for Cat 5 UTP). The longer the cable, the more propagation delay; the slower the nominal velocity of propagation (NVP) of the cable, the more the propagation delay. Third, latency is added according to which networking devices -- whether they be Layer 1, 2, or 3 (and how they are configured) -- are added in the path between the two communicating computers. The actual transmission time (the duration of the host actually sending bits) must also be included in understanding timing on networks. Latency does not depend solely on distance and number of devices. For example, if three properly configured switches separate two workstations, the workstations may experience less latency than if two properly configured routers separated them. This is because generally routers conduct more complex and time consuming decision-making functions (recall that routers are layer 3 devices and look at more encapsulation data than layer 2 switches do).
| Web Links
| Cisco Connection Online
| Cisco Documentation
| Search Cisco